Directional couplers are widely used in front end module (FEM) products, such as in radio transceivers, wireless handsets, and the like. For example, a directional coupler can be used to detect and monitor electromagnetic (EM) output power. Additionally, when a radio frequency (RF) signal generated by an RF source is provided to a load, such as to an antenna, a portion of the RF signal can be reflected back from the load. An EM coupler can be included in a signal path between the RF source and the load to provide an indication of forward RF power of the RF signal traveling from the RF source to the load and/or an indication of reverse RF power reflected back from the load. EM couplers include, for example, directional couplers, bi-directional couplers, multi-band couplers (e.g., dual band couplers), and the like.
Referring to FIG. 1, an EM coupler 100 typically has a power input port 102, a power output port 104, a coupled port 106, and an isolation port 108. The electromagnetic coupling mechanism, which can include inductive or capacitive coupling, is typically provided by two parallel or overlapped transmission lines, such as microstrips, strip lines, coplanar lines, and the like. A main transmission line 110 extends between the power input port 102 and the power output port 104 and provides the majority of the signal 116 from the power input port 102 to the power output port 104. A coupled line 112 extends between the coupled port 106 and the isolation port 108 and may extract a portion 114 of the power traveling between the power input port 102 and the power output port 104 for various purposes, including various measurements. When a termination impedance is presented to the isolation port 108, an indication of forward RF power traveling from the power input port 102 to the power output port 104 is provided at the coupled port 106.
In a forward coupling mode, as in FIG. 1, the portion 114 is a fraction of the main signal 116 RF power traveling from the power input port 102 to the power output port 104. EM couplers are typically rated by their coupling factor, usually stated in decibels, which is a measure of the ratio of the power of the portion 114 coupled from the power of the input signal 116. For example, a 20 dB coupler will provide a coupled signal, e.g., a portion 114, that is 20 dB lower than the input power, or about 1% of the input power.
It is generally desirable to have a relatively low coupling factor to not overly remove power from the main signal, but it is also desirable for the coupling factor to be certain and consistent, to allow accurate assessments of the power of the main signal.